||Most mutualisms are exploited by one or more third-party "interlopers" that intercept the goods and services exchanged between partners. I investigated the ecology of one such exploiter of the Mesoamerican ant-acacia mutualism, the jumping spider Bagheera kiplingi. Systematic observations of the spider in coastal Quintana Roo, Mexico, showed that its principal diet comprised Beltian bodies, specialized leaf tips exchanged between swollen-thorn acacias ( Vachellia collinsii ) and their coevolved Pseudomyrmex ant-inhabitants. Analysis of δ 15 N and δ 13 C isotopic profiles of system components confirmed the spider's herbivory: individuals sampled derived approximately 98% of assimilated C and N from its host plant (90% directly from plant tissue, 8% indirectly from ant brood). The known range of B. kiplingi throughout Mesoamerica coincides with that of ant-acacias. The results of this study suggest that commodities modified for trade in a pairwise ('closed-market') mutualism may redirect the evolutionary trajectory of the individuals that intercept them.